1. Field of the Invention
This invention relates to electronic music synthesis and in particular is concerned with the assignment of a limited number of tone generators to actuated keyboard switches.
2. Description of the Prior Art
The current trend to use microelectronic implemented tone generators for keyboard operated musical instruments has led to the development of systems which achieve a measure of economy by using a plurality of tone generators which is less in number than the number of keyboard switches in the keyboard array of keyswitches. An assignment logic is implemented to assign the members of the available tone generators to keyswitches as they are depressed to their actuated keyswitch states. An inevitable resource availability problem arises when all the tone generators have been assigned to keyswitches and an additional keyswitch is actuated.
There have been several assignment systems implemented to cope with the resource availability problem. One of the earliest assignment systems was disclosed in U.S. Pat. No. 2,577,493 entitled "Electronic Musical Instrument." In the disclosed system a number of analog oscillators are assigned by means of a multicontact set of keyswitch contacts to actuated keyswitches. The assignment action also introduces a preselected capacitor so that the assigned oscillator generates a signal whose fundamental frequency corresponds to the assigned actuated keyswitch. The assignment system disclosed in the reference patent does not teach any means to cope with the situation in which all the tone generators have been assigned and an additional keyswitch is actuated.
Because there are ten fingers available to the musician, it would seem at first thought that ten tone generators would suffice for any one keyboard. Modern keyboard instruments have incorporated a tone effect that demands more than one tone generator to be assigned to action of a single finger. This tone effect is given the generic name of "sustain." Unfortunately this term has led to some confusion because the present usage of envelope modulation is to refer to an ADSR (attack/decay/sustain/release) time envelope modulation function. More properly the older term of "sustain" should now be called "long release." With a long release, it is a fairly easy matter to release a keyswitch and actuate another keyswitch with a single finger while the first tone generator is still operative and is automatically being reduced in tone volume by its ADSR envelope modulation function.
A tone generator assignment system intended to accommodate the resource availability problem when a long release mode has been implemented for a specified keyboard is described in U.S. Pat. No. 3,610,806 entitled "Adaptive Sustain System For Digital Electronic Organ." In the disclosed system the tone generators follow the normal sequence of assignment of attack, decay, and release as long as an unassigned tone generator is idle and is available for assignment. When all the tone generators have been assigned, the system automatically enters an adaptive sustain mode in which any tone generator assigned to a note associated with a key on the manual having a long release effect, and which generator is supplying the waveform that has the longest duration of its envelope modulation release phase, is switched immediately from a long release to a relatively shorter release while still employing exactly the same predetermined envelope modulation function.
A problem that is inherent with tone generator assignment logic systems is that they are not ideal in the sense that fairly common musical systems arise in which the assignment logic operates in a manner which is contrary to that expected by the musician.